KM-I-1: Water temperature in the sea | Umweltbundesamt

KM-I-1: Water temperature in the sea

The annual mean surface temperature of the North Sea has been rising significantly since 1969. For example, the mean warming amounts to 0.26 °C per decade. The annual values are subject to natural fluctuations.

The North Sea is getting warmer

Global warming is reflected not just in rising air temperatures – the seas and oceans are warming increasingly too. Especially in years with persistent heat waves, water temperatures rise distinctly. As a result, some extraordinarily high water temperatures were recorded on Germany’s coasts in the exceptionally hot summer of 2003. In August of that year, the mean surface temperature measured across the entire North Sea fell short of the August 1997 temperature of roughly 17.6 °C by just 0.2 °C.⁶⁸ In some locations and for individual days, temperatures may have been even higher. In the hot summer of 2018, the maximum temperatures measured at coastal gauging stations in the North Sea and the Baltic Sea rose as a high as 26 °C, thus reaching a Mediterranean level⁶⁹. Latterly, the 2022 water temperatures in the North Sea broke the heat records. In some places, the surface mean temperature in summer amounted to more than 1 °C above the long-term summer mean of 1997 to 2021⁷⁰.

Their storage, buffering and exchange functions give the Earth’s oceans a central role in its climate system. More than 90 % of the additional heat energy generated by the anthropogenic greenhouse effect is stored in the oceans. Contrary to the air temperatures above the mainland, which are more exposed to the influence of localised weather patterns as well as natural climate variation, the oceans’ temperature development indicates less pronounced fluctuations, especially in deeper strata. Between 1960 and 2020 the heat energy contained in the upper 2,000 metres of the earth’s oceans increased by 380±81 sextillion Joule (380±81 x 1021 J⁷¹). The mean warming rate for the entire surface of the Earth amounts to 0.39±0.08 Watt per square metre (W/m2). On detailed inspection it became clear that over the past roughly 30 years, the warming of the ocean proceeded more strongly than when records began. The mean of the heat energy absorbed annually in the period of 1986 to 2020 was approximately 8 times higher than between 1958 and 1985⁷². In recent years, the world’s oceans have absorbed more heat energy every year than in the preceding year.

The warming of the seas and oceans does not just take place at the surface, because the heat energy absorbed is transferred into the innermost parts of the sea, reaching even deeper layers, down to the sea floor⁷³. It therefore follows that measuring global warming primarily in terms of the increase of the globally calculated mean of the surface temperature is tantamount to a gross underestimation and misperception of this complex issue. In fact, global warming came into question when the increase in the surface temperature in the period of 1998 to 2013 more or less stagnated (global warming hiatus)⁷⁴.

Evidence for the warming of North Sea and Baltic Sea is found in the large-scale analyses of surface temperatures which have been carried out in the North Sea for over 50 years by the Federal Maritime and Hydrographic Agency (BSH). For the indicator, the annual mean temperatures were calculated on the basis of aggregated measurements. Statistically speaking, a significant linear trend is discernible for the entire period in question. It is notable that the linear mean of the temperature increase amounting to roughly 0.26 °C per decade is overlaid with fluctuations in various timescales. This is caused by various natural variability patterns such as the Atlantic Multidecadal Oscillation. The time series of the annual mean air temperature in Germany shows a similar development. Similar variability phenomena can also be observed in other parts of the world. Given the development of the mean surface temperatures along the North American Pacific coast well into the central Pacific ocean, it can be assumed that a warming regime has existed in that part of the Pacific since 2014.^{75 75}

The highest annual mean temperatures of the North Sea so far – typically resulting from extreme warming in the summer months – amounted to 11.0 °C (2003, 2006, 2016, 2020) and above (11.4 °C, 2014), and latterly 11.2 °C (in 2022).

The consequences of warming in the North Sea and the Baltic Sea as well as other seas in the world with regard to marine ecosystems have been documented in various scientific studies⁷⁶. Species either adapt their range of distribution or become extinct – either locally or regionally (cf. Indicator FI-I-1). Indirect side effects of climate change such as lack of oxygen and the acidification of the seas also contribute to changes in the diversity, composition and distribution of species thus changing the entire food web prevailing in marine habitats. The economic impacts on deep-sea fisheries are difficult to distinguish (cf. Indicator FI-I-2). As far as German coastlines are concerned, high seawater temperatures have made headlines in recent years when raised concentrations of vibrios created health hazards for bathers (cf. Indicator GE-1-7) or when bathing tourism was affected by blue-green algal bloom. An additional driver of these developments is the persistent eutrophication via nutrient inputs from rivers. The maximum nitrogen concentrations at the limnic-marine transition point stipulated for Germany, are nowadays complied with in respect of the mean for all rivers flowing into the North Sea, but they are still being exceeded with regard to the Baltic Sea. Some of the tributaries to the North Sea and the Baltic Sea still indicate high concentrations.

A direct consequence of heat storage in the oceans is the expansion (increase in volume) of seawater which is one of the crucial causes of sea level rise (cf. Indicator KM-I-2). In 2021 the global sea level was 97 mm above the 1993 level (when satellite measuring began) thus reaching a record level. Just under 40 % of this increase is due to the thermal expansion of seawater, most of the rest of the increase in mass is due to melt water inflows.⁷⁷